Continuous wet grinding of solid materials in tube mills



April 8, 1969 E. LEHRER ETAL 3,437,275 CONTINUOUS WET GRINDING 0F SOLID MATERIALS IN TUBE MILLS Filed April 14, 1966 FIG. 2

INVENTORS: ERWIN LEHRER JOSEF WOELLHAF BY ATT'YS.

United States Patent ()flice 3,437,275 Patented Apr. 8, 1969 Int. cunoze 17/00 US. Cl. 241-15 4 Claims ABSTRACT OF THE DISCLOSURE A process for the continuous wet grinding of solids in liquids by means of a compartmented tube mill having grinding balls. The premixed feed is introduced together with a gas current into the rotating mill and forced by the gas through openings in the compartment partitions using a pressure drop of from to 500 kg./sq. m. between compartments.

This invention relates to the continuous wet grinding of solid materials or the dispersion of powdered su' stances in liquids in tube mills. The invention relates particularly to a wet grinding method in which the material being treated is forced through a compartmented tube mill by means of a current of gas.

Methods are already known for the continuous wet grinding of solids or the dispersion of powdery substances in liquids. Thus for example lacquers have been prepared by means of tube mills. These are similar to ball mills, except that tube mills have a greater length-to-diameter ratio, and are subdivided into compartments by partitions to which the premixed feed material is continuously supplied at one end through a bore in the shaft. The material is passed from one compartment to the next through openings in the partitions and the finished product withdrawn from the last compartment through the shaft. The compartments are partly filled with grinding balls.

These methods have great advantages because they are continuous. It has been found however that the action of tube mills is far less satisfactory than that of 'ball mills operated batchwise. If the openings in the partitions are made small, the individual compartments become completely filled with process material. The grinding or dispersing effect of the mill is thus considerablyimpaired. If the openings are made too large, the compartments do not fill completely but exchange of material from one compartment to the next takes place so rapidly that inadequately ground or dispersed portions pass into the last compartment and are discharged. The quality of the product may thus be impaired.

The invention has for its object to avoid completely these disadvantages in the continuous wet grinding of solids or dispersion of powdered substances in liquids by means of tube mills subdivided into compartments by means of a plurality of partitions and provided with grinding balls.

According to this invention, premixed feed is introduced by means of a current of gas into the rotating grinding chamber and forced by the gas current through openings provided approximately in the middle of the partitions and the finished product discharged from the mill.

It has been found to be advantageous to saturate the gas current, prior to impingement on the process material, with vapor of the liquid present during the grinding or dispersion. This is particularly necessary when using low boiling point solvents in order to avoid incrustation of the point of entry of the gas current.

The tube mill for carrying out the process according to this invention has a motor driven casing mounted at both ends and compartmented by a plurality of partitions provided 'with openings and according to this invention the openings in the partitions of the compartments are of such a size that a pressure difference of from 10 to 500 kg./sq. In. is required to force the material from one compartment to the next.

The openings in the partitions between the compartments are advantageously formed as slots whose width is less than the diameter of the balls. The balls therefore cannot pass from one comparment to another. Similar results are achieved with openings provided with a grate or screen whose mesh is less than the diameter of the balls.

A tube mill for carrying out the process according to the invention is shown diagrammatically by Way of example in FIGURE 1 of the accompanying drawings. A tube 1 is closed at the ends by covers 2 and 3 which are provided withbored shafts 4 and 5. These are mounted in bearings 6 and 7.

The tube lmay be rotated by a motor 8 through a drive 9. The tube 1 is subdivided by partitions 10 to 13 into compartments 14 to 18 which are partly filled with grinding balls 19. Process material 20 is between and above the balls.

Provided approximately in the middle of the partitions 10 to 13 are openings 21 whose most favorable size may be determined in a preliminary experiment. For this purpose for example an amount of material corresponding to the capacity of the tube mill and preferably having flow properties corresponding to the mean value is pumped through the opening to be tested. The size of the opening is then varied until the pressure ditference between two measuring points immediately before and after the opening has the desired magnitude. About 50 kg./sq. m. has been found to be the most favorable value, but this need not be strictly adhered to. It does however guarantee that even if changes occur in the operating conditions the pressure loss will remain within the range of 10 to 500 kg./sq. m. which has been found to be favorable.

A feed pipe 22 is introduced into the bore of the shaft 4 using a stufiing box 23 to make a pressure-tight joint. The feed is pumped through the tube 22 into the tube mill and at the same time a gas current is introduced which enters through the tube 24, is measured with a flowmeter 25 and is saturated in a container 26 with vapor of the liquid in which grinding or dispersion is to be carried out. The process material and the gas pass from compartment to compartment through the openings 21, the material being progressively ground. The finished product leaves the mill through bore 27 in shaft 5.

The amount of gas supplied need not be kept constant; it may vary within wide limits. Good results are achieved with a gas current of the same volume as that of the process material.

An advantageous shape of openings is shown in FIG- URE 2 of the accompanying drawings. These openings consist of parallel slots 28 whose width is less than the diameter of the grinding balls and whose total area may be determined in the manner described above.

Similar results are obtained with an embodiment according to FIGURE 3 of the accompanying drawings. The

' circular opening 29 is covered with a grate 30 whose mesh is less than the diameter of the grinding balls.

Exhaustive tests have shown that very good and consistent results are obtained in a tube mill by the method according to this invention. Dispersions of magnetic 'y iron oxide in a solution of a binder which have been prepared in this wall will completely satisfy the exacting requirements made for example in the manufacture of magnetic recording tape. In continuous operation, the material builds up in the individual compartments only to the level of the upper edge of the openings 21. Inadequately ground or dispersed portions cannot be detected in the ground product. The performance, with reference to the volume of the mill, is much higher than that of a batch-operated ball mill, and the expenditure for maintenance is considerably less.

We claim:

1. A process for the continuous wet grinding of solids for the continuous dispersion of powdery substances in a liquid by means of a tube mill subdivided into compartments by a plurality of partitions and having grinding balls which comprises: introducing a premixed liquidsolid feed together with a gas current into the first rotating grinding compartment of the tube mill; forcing said feed through openings provided approximately in the middle of said partitions by means of a pressure difference from one compartment to the next; and discharging said feed from said mill after said feed has passed the last grinding compartment of said mill.

2. A process for the continuous wet grinding of solids for the continuous dispersion of powdery substances in a liquid by means of a tube mill subdivided into compartments by a plurality of partitions and having grinding balls which comprises: introducing a premixed liquidsolid feed together with a gas current into the first rotating grinding compartment of the tube mill, said gas current being saturated with the vapor of said liquid prior to impingement of said current on said feed; forcing said feed through openings provided approximately in the middle of said partitions by means of a pressure difference from one compartment to the next; and discharging said feed from said mill after said feed has passed the last grinding compartment of said mill.

3. A process as claimed in claim 2 wherein a pressure ditference of from 10 to 500 kg./ m. on opposite sides of the partitions is exerted on the premixed feed by means of the current of saturated gas.

4. A process as claimed in claim 2 wherein the volume of the current of saturated gas is about equal to the volume of the process material supplied to the tube mill.

References Cited UNITED STATES PATENTS 230,660 8/1880 Robertson 241-- 1,591,941 6/1926 Newhouse 241-45 HARRISON L. HENSON, Primary Examiner.

US. Cl. X.R. 241-45 

